Send notification to Skeleton on Proxy destruction to "Unsubscribe" method.

[Tejveerpratap2803]

Notify Skeleton on Proxy destruction to unsubscribe method resources

When a Proxy is destroyed, TeardownMethods() now sends an UnsubscribeServiceMethod IPC message to the Skeleton, which removes the proxy's shared memory region and unregisters all method call handlers for that proxy.

• Added UnsubscribeServiceMethod IPC path (local + remote) and RegisterOnServiceMethodUnsubscribedHandler to IMessagePassingService
• Skeleton registers the unsubscription handler in PrepareOffer; on notification, it calls MethodResourceMap::Remove and unregisters all associated method call handlers
• Added MethodUnsubscriptionRegistrationGuard RAII guard to manage handler lifetime; destroyed in PrepareStopOffer
• Added SkeletonMethod::OnProxyMethodUnsubscribeFinished to complete the unsubscription flow
• Refactored registration_guards_ from an ApplicationId-keyed map (with inner vector of guards) to a flat unordered_map<ProxyMethodInstanceIdentifier, pair<pid_t, MethodCallRegistrationGuard>> — one entry per proxy method instance; unsubscribe now erases by the full ProxyMethodInstanceIdentifier, preventing accidental removal of entries from other proxies sharing the same ApplicationId, while crash-detection cleanup still iterates by application_id extracted from the key to correctly remove all stale entries from a crashed process
• Added unit tests covering all new IPC paths, the unsubscription flow, and the two-proxies-from-the-same-application scenario